They pointed a gun at him and his son, he was armed and took them out, everything good except maby the son seeing their heads getting blown off and of course the hearing.
It's not uncommon that armed robbers kill people, perhaps he has even seen that being a cop and all.
Sure perhaps he could have done it better and put the kid down or something but i think it's one of those in the moment kind of things when they point a gun at him and his son.